The present invention relates generally to apparatus and methods for extruding thermoplastic material and, more particularly, to an apparatus for swingable mounting of a gear pump in a thermoplastic extrusion line of the type having a gear pump interposed between a melt extruder and an extrusion die.
Many various forms of thermoplastic polymeric material are conventionally extruded into a wide variety of products including sheets, film, rods, tubes, and strands, as well as various other products. Basically, the extrusion process involves melting of the, thermoplastic material employed, and sufficient pressurization of the melted material to cause it to flow, at a desired rate, through an appropriate die to form the intended end product. An important determination of the quality of the end product is its conformity, within acceptable tolerances, to predetermined dimensional and weight specifications. As a general principle, the quality of an extruded product tends to decrease as the extrusion rate increases. Over the years, it has therefore been an ongoing problem in the industry in striking an acceptable balance between an economical rate of extrusion and an acceptable quality of the extruded product.
Conventional apparatus for extruding thermoplastic material typically includes a cylindrical barrel in which is rotatably disposed a conveying screw. The thermoplastic material is fed in a solid pellet or powder form into the barrel at one end and conveyed to the other end of the barrel by rotation of the screw. The heat required to transform the thermoplastic material from a solid at room temperature to a molten material at the desired extrudate temperature is typically generated by equipping the extruder with heating elements.
Conventional extruders are widely recognized to be disadvantageously subject to undesirable deviations in the quality of the extruded product. While the volumetric input of thermoplastic material to the extruder and the operational parameters of the extruder, e.g., rotational speed of the screw and temperature of the extruder heaters, may be maintained substantially constant, variations in the thermoplastic material such as its density and viscosity, as well as variations in the operating conditions of the extruder such as uneven temperature distribution and backpressurization, cause unacceptable variations in the quality of the extruded product exceeding established tolerances. When such variations result in an extruded product exceeding established thickness specifications by more than the acceptable tolerance, the extruder utilizes an excessive amount of thermoplastic material and, in turn, the profitability of the extruder suffers. When such variations result in an extruded product of a thickness less than established thickness specifications by more than the acceptable tolerance, the product is defective and unsalable.
In recent years, various techniques and approaches have been developed to attempt to control deviations in the output of extrusion systems to avoid these problems. One such approach is to utilize a gear pump following the extruder to meter the delivery of the molten thermoplastic material to the forming die. The basic construction and operation of gear pumps is well known, basically including a pair of counter-rotating intermeshing gears driven within a pump housing to transport the molten thermoplastic material in a positive displacement manner by entrapping the material between the teeth of the gears. As a result, the volumetric output of a gear pump is a function of its operational speed and is substantially unaffected by pressure surges and fluctuations ill the input flow of thermoplastic material from the extruder. As a result, the use of a gear pump reduces variations in the quality of the extruded product, both on short term and long term bases, by delivering a substantially uniform volumetric flow of thermoplastic material to the die of the extrusion system. Representative examples of gear pump-assisted thermoplastic extrusion lines are disclosed in U.S. Pat. Nos. 3,649,147; 4,013,745; 4,101,613; 4,209,476; 4,452,750; 4,478,775; 4,501,498; 4,663,103; and 5,122,315.
As persons skilled in the art will recognize, one of the important factors affecting the proper functioning of a gear pump in such an extrusion line is that the gear pump must be mounted in precise proper alignment with the output end of the melt extruder and the input side of the die. Typically, the gear pump is driven by its own individual electric motor through a gear reducer unit which enables the speed of the gear pump to be selectively adjusted in relation to the particular process requirements of the extrusion line. Because of the need to maintain proper alignment of the gear pump, it has been common practice in the past to mount the gear pump/reducer unit/drive motor assembly to its own respective base plate or frame which, in turn, is securely mounted to the floor between the melt extruder and the die once the gear pump has been properly aligned therewith during set-up of the extrusion line, thereby to best ensure that the alignment of the gear pump is maintained during the course of processing. Because periodic maintenance or servicing of the gear pump assembly may be necessary, one alternative mounting system which has been employed in recent years is to utilize a wheeled cart having lockable casters on which the gear pump, its drive motor, and gear reducer unit may be mounted. The cart enables the gear pump to be disassembled from the extruder and the die when necessary so that the entire cart structure can be simply wheeled out of the extrusion line for maintenance or other servicing. While this cart mounting arrangement improves the convenience of maintenance and servicing of the gear pump assembly, it still suffers the same disadvantage of fixed gear pump mounting systems, namely, that the gear pump must be precisely realigned with the output end of the extruder when the gear pump is returned to service, just as during the original set-up of the extrusion line, which can be a time consuming task and necessarily involves a corresponding loss of extrusion production.